Radio communication system

ABSTRACT

A radio communication system has a random access channel for the transmission of data ( 214 ) from a secondary station to a primary station. Such a channel is intended for use by secondary stations having data ( 214 ) to transmit to a primary station while not actually engaged in a call.  
     By enabling access requests ( 202 ) to be transmitted with a greater range of possible signatures, a much greater number of degrees of freedom is available to a secondary station requesting access to a random access channel. This enables significantly improved efficiency of resource allocation by increasing the amount of information transmitted to the primary station by the access request ( 202 ).

The present invention relates to a radio communication system having arandom access channel for transmissions from a secondary station to aprimary station, and further relates to primary and secondary stationsfor use in such a system and to a method of operating such a system.While the present specification describes a system with particularreference to the emerging Universal Mobile Telecommunication System(UMTS), it is to be understood that the techniques described are equallyapplicable to use in other mobile radio systems. In this specificationthe term random access packet channel refers to the logical channel onwhich random access packet transmissions take place, which wouldtypically consist of a number of distinct physical channels.

A random access channel is a normal component of a radio communicationsystem, enabling a Mobile Station (MS) to send short messages to a BaseStation (BS). Applications include signalling to the BS when the MS isturned on, sending a packet of data to the BS when the MS may not beengaged in a call, and requesting the BS to allocate a resource for theMS to use.

In a system where mobile stations often have a requirement to sendpackets of data to the BS when not actually engaged in a call it isadvantageous to provide a random access packet channel with similarcharacteristics to a standard random access channel but intended for thetransmission of small and medium sized packets from a MS to the BS.

In an embodiment of a such a scheme developed for UMTS, there are anumber of random access packet channels available to a MS. A request foraccess to a packet channel resource sent by the MS is encoded with oneof 16 available signatures. Each signature corresponds to a request fora particular resource required by the MS, for example a channel having aparticular bit rate. If a suitable resource is available for use, the BSallocates it to the requesting MS.

A problem with such a scheme is the limited amount of information thatcan be transferred during the access procedure, because of the limitednumber of signatures and channels available. This problem reduces theefficiency of resource allocation in the system.

An object of the present invention is to provide a random access packetchannel having improved resource allocation.

According to a first aspect of the present invention there is provided aradio communication system having a random access channel fortransmissions from a secondary station to a primary station, thesecondary station having means for requesting allocation of a randomaccess channel resource by transmission of an access preamble encodedwith a first signature selected from a first set of signatures and theprimary station having means for receiving the access preamble, fordetermining the first signature and for transmitting an accessacknowledgement encoded with a second signature selected from a secondset of signatures indicating whether the requested resource isavailable, wherein the choice of first signature provides furtherinformation regarding the resource allocation request and wherein eachsignature of the second set corresponds to a plurality of signatures inthe first set.

According to a second aspect of the present invention there is provideda primary station for use in a radio communication system having arandom access channel for transmissions from a secondary station to theprimary station, wherein means are provided for receiving an accesspreamble transmitted by the secondary station, which preamble is encodedwith a first signature selected from a first set of signatures, fordetermining the first signature, for determining from the accesspreamble which random access channel resource the secondary stationrequests to be allocated and for transmitting an access acknowledgementencoded with a second signature selected from a second set of signaturesindicating whether the requested resource is available, wherein thechoice of first signature provides further information regarding theresource allocation request and wherein each signature of the second setcorresponds to a plurality of signatures in the first set. According toa third aspect of the present invention there is provided a secondarystation for use in a radio communication system having a random accesschannel for transmissions to a primary station, wherein means areprovided for requesting allocation of a random access channel resourceby transmission of an access preamble encoded with a first signatureselected from a first set of signatures and for receiving an accessacknowledgement encoded with a second signature selected from a secondset of signatures indicating whether the requested resource isavailable, wherein the choice of first signature provides furtherinformation regarding the resource allocation request and wherein eachsignature of the second set corresponds to a plurality of signatures inthe first set.

According to a fourth aspect of the present invention there is provideda method of operating a radio communication system having a randomaccess channel for transmissions from a secondary station to a primarystation, the method comprising the secondary station requestingallocation of a random access channel resource by transmission of anaccess preamble encoded with a first signature selected from a first setof signatures and the primary station receiving the access preamble,determining the first signature and transmitting an accessacknowledgement encoded with a second signature selected from a secondset of signatures indicating whether the requested resource isavailable, wherein the choice of first signature provides furtherinformation regarding the resource allocation request and wherein eachsignature of the second set corresponds to a plurality of signatures inthe first set.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a block schematic diagram of a radio communication system;

FIG. 2 illustrates a basic random access channel scheme;

FIG. 3 illustrates an enhanced random access channel scheme in which thetransmission time of preamble and contention resolution preambles may beoffset; and

FIG. 4 is a flow chart illustrating a method in accordance with thepresent invention for attempting to access a random access packetchannel using timing offsets.

In the drawings the same reference numerals have been used to indicatecorresponding features.

Referring to FIG. 1, a radio communication system comprises a primarystation (BS) 100 and a plurality of secondary stations (MS) 110. The BS100 comprises a microcontroller (μC) 102, transceiver means (Tx/Rx) 104connected to antenna means 106, power control means (PC) 107 foraltering the transmitted power level, and connection means 108 forconnection to the PSTN or other suitable network. Each MS 110 comprisesa microcontroller (μC) 112, transceiver means (Tx/Rx) 114 connected toantenna means 116, and power control means (PC) 118 for altering thetransmitted power level. Communication from BS 100 to MS 110 takes placeon a downlink channel 122, while communication from MS 110 to BS 100takes place on an uplink channel 124.

A basic scheme for a random access packet channel operating in afrequency division duplex system is shown in FIG. 2, with the uplinkchannel 124 drawn above the downlink channel 122. In an access phase,the MS 110 first transmits a preamble (P) 202 encoded with one of 16possible signatures at a low power level in a particular access slot. Asignature is a signal characterised by its scrambling code andchannelisation code modulated by a specific bit sequence. A mutuallyorthogonal set of signatures can be obtained by defining a set ofmutually orthogonal bit sequences for the modulation. Hence, a differentset of signatures can be obtained by changing the scrambling code or thechannelisation code (i.e. the physical channel), or by using a differentmutually orthogonal set of bit sequences. In the basic system shown inFIG. 2 there is a one to one mapping between the preamble signature andthe acknowledgement (206), such that receipt of an acknowledgement bythe MS 110 uniquely determines which signature is being acknowledged.Although the present specification refers to sets of 16 signatures,different implementations may use sets having different numbers ofsignatures.

Each of the available signatures is mapped to a single bit rate for thepacket channel. The set of available signatures, and mappings betweensignatures and bit rates, may be predetermined or may be regularlybroadcast by the BS 100. The MS 110 selects a signature for encoding thepreamble 202 corresponding to its required bit rate. If there is morethan one signature available corresponding to the required bit rate, theMS 110 selects one at random. If the BS 100 receives and decodes thepreamble correctly it transmits a preamble acknowledgement (A) 206. Inthe example shown in FIG. 2, after the first preamble 202 is transmittedno acknowledgement is returned in the slot 204 allocated for it (whichmight for example be 1 ms in length). The MS 110 therefore transmitsanother preamble 202 at a higher power level. This is received anddecoded by the BS 100, which transmits an acknowledgement 206 andthereby completes the access phase.

As well as informing the MS 110 that its preamble 202 has been received,the acknowledgement 206 may be positive, to signal that the requestedresource is available, or negative, to signal that it is in use andaccess is denied to the MS 110. A negative acknowledgement (NACK) may beindicated by the BS 100 inverting the phase of the signature (withrespect to some reference or pilot signal). Alternatively, some of thesignatures used by the BS 100 for acknowledgement may also be used as aNACK.

The BS 100 will only transmit one acknowledgement for each access slot,however many preambles 202 were transmitted. One basis for the selectioncould be to acknowledge the preamble 202 received with the highestpower. The initial power level at which a MS 110 transmits the preamble202 is typically determined by the MS 110 using open loop power control,so that a MS 110 is not at a disadvantage compared to another MS 110nearer to the BS 100. If more than one preamble 202 was transmitted buteach preamble was encoded with a different signature then each MS 110will know whether or not its preamble 202 was received correctly.However, it is possible that more than one MS 110 selected the samesignature, and therefore believes that its preamble 202 has beenreceived. If each of these mobile stations 110 begins to transmit itsdata the result will be a collision, with none of the data likely to bereceived correctly.

To reduce the chances of this happening, a contention resolution phasemay follow the transmission of an acknowledgement 206 which indicatedthat the requested resource was available. Each MS 110 which transmitteda preamble 202 encoded with a signature corresponding to thatacknowledged by the BS 100 now transmits a further contention resolutionpreamble (CP) 208. This preamble 208 is encoded with a signaturerandomly selected from another set of 16 possible signatures. This setmay be different from the set used for the access preamble 202 (eitherby changing the set of modulating bit sequences, the scrambling code orthe channelisation code), or alternatively the set of signatures may beshared between access and contention resolution phases. The BS 100 thenissues a contention resolution acknowledgement (CA) 210 corresponding tothe selected preamble 208, for example that received with the highestpower, which acknowledgement 210 enables the MS 110 to transmit itsdata. Hence, if more than one MS 110 selected the same access preamble202 the chance of the same contention resolution preamble 208 also beingselected is small. A channel assignment message may optionally betransmitted at substantially the same time as the contention resolutionacknowledgement 210, or even as part of the acknowledgement 210, asdisclosed in our co-pending International patent applicationPCT/EP00/06988 (our reference PHGB 000003).

After this contention resolution phase the BS 100 begins transmission ofa Physical Control CHannel (PCCH) 212, which includes power controlinformation to instruct the MS 110 to adjust its transmission power asnecessary, and the MS 110 transmits one or more data packets (PKT) 214on the allocated packet channel, which is normally on a differentphysical channel to those used for the preamble transmissions. The PCCH212 may begin simultaneously with the transmission of the data 214, ormay precede it sufficiently for closed loop power control to beestablished before the data transmission.

A particular problem with the basic scheme described above is that theefficiency of resource allocation is limited by the number of choicesavailable for the access preamble 202. In an embodiment of the schemefor UMTS, for example, there are 16 signatures and 12 random accesssub-channels available, giving a total of 196 degrees of freedom.

This problem is solved in a system made in accordance with the presentinvention by defining a larger set of signatures, which may have lowcross correlations rather than strict orthogonality. Such a set ofsignatures could be defined by modifying the bit sequences used todefine the signatures or by using different scrambling codes, or by acombination of both techniques. In order to avoid the need to allocate alarger number of downlink channelisation codes, the same acknowledgement(206) in FIG. 2 may be transmitted in response to any one of a definedset of preamble signatures. This may increase the probability ofcollisions, but typically this would be an acceptable drawback comparedthe other advantages of the system.

The availability of many more degrees of freedom enables significantlymore efficient resource allocation. For example, in the case of channelassignment it enables there to be more bit rates.

A system made in accordance with the present invention can make use ofthe much greater number of degrees of freedom in a number of ways.Possible assignments for each combination of signature, channel and(non-zero) timing offset include the following (either singly or incombination):

-   -   as a request for a particular bit rate;    -   as a request for a particular access class (with associated        priority);    -   as a request for additional resources (e.g. a downlink shared        channel);    -   for indicating the priority of the message.    -   for indicating the length (or minimum or maximum length) of the        packet to be sent;    -   for use by a specific MS 110; and    -   for use by MSs 110 using one of a set of common higher layer        messages.

Some of these assignments could reduce the quantity of higher layerinformation required to be included in the data packet 214. For example,if a particular MS 110 is assigned the exclusive use of a subset of thepossible uplink signals, its identity is automatically known by the BS100 and need not be included in the data packet 214. Such a reduction inoverhead on packet transmission could be particularly useful for lowdata rate applications. The same principle could be applied to othercommonly-used items of higher layer information. Other assignments couldimprove system responsiveness. For example, indicating the minimumlength of a packet to be sent enables the BS 100 to send an ARQ(Automatic Repeat reQuest) to the MS 110 if it does not receive at leastthe indicated minimum length. Indicating the maximum length of a packetmay enable the BS 100 to improve its planned resource allocation.

FIG. 3 illustrates how the set of possible signatures can be extended byintroducing different time offsets. The time of transmission of theaccess preamble 202 may be offset by T_(off) with respect to theboundary 302 of the access slot (which is itself defined relative totiming signals transmitted by the BS 100). The access preamble 202 andcontention resolution preamble 208 both comprise 4096 chips, while thelength of the access slot is 5120 chips. By allowing timing offsetsT_(off) of multiples of 256 chips, up to 19 different non-zero values ofT_(off) are possible without introducing ambiguity about which slotcontained the preamble 202,208. When T_(off)=0 the behaviour of thesystem is identical to that of a system without the possibility oftiming offsets.

The timing offset T_(off) preferably advances the transmission time ofthe preambles 202,208, since any delay in its transmission might meanthat the BS 100 is unable to detect the preamble in time to transmit anaccess acknowledgement 206 in an appropriate time slot.

The timing offset for the contention resolution preamble 208 ispreferably the same as that for the access preamble 202, so that the BS100 can readily identify the two preambles 202,208 as coming from thesame MS 110.

It is preferable for there to be no timing offsets in the downlinkchannels 122, so that the acknowledgement 206 of an access preamble 202is the same irrespective of the timing offset used. This enablescollisions between offset and non-offset transmissions to be resolvedduring the contention resolution phase. The use of timing offsets hasthe further advantage of increasing the number of different signalsavailable for use during the contention resolution phase, therebyreducing the probability of unresolved collisions.

As a variation on the above scheme, the preamble signature could actpurely as an identifier for the access attempt, while the informationrelating to the resource allocation request could be carried by thetiming offset and sub-channel selected by the MS 110.

A flow chart summarising a method in accordance with the presentinvention, including the use of time offsets, for a MS 110 attempting toaccess a packet channel resource is shown in FIG. 4. The method starts,at step 402, when the MS 110 has data for transmission on the randomaccess packet channel. The MS 110 firstly, at step 404, selects asignature corresponding to the required resource, for example therequired bit rate. Next the MS 110, at step 406, determines anappropriate timing offset T_(off), for example to indicate its identityor the urgency of the data for transmission. Having selected theseparameters, the MS 110, at step 408, attempts to access the packetchannel by transmitting one or more access preambles 202 and acontention resolution preamble 208, as described above.

In alternative embodiments, not all the features shown in FIG. 3 may beneeded. For example, inner loop uplink power control may not beessential, so the PCCH 212 may not be present. The collision resolutionpart may not be essential, in which case the contention resolutionpreamble 208 and acknowledgement 210 messages may not be present. Theconsequences of these changes would be simpler implementation, but atthe cost of poorer Eb/No performance and increased collision probabilityof message parts. In some cases, all the required signalling informationmay be conveyed by the choice of access sub-channel and signature, inwhich case the message part would not be needed and could be omitted. Asan alternative to selecting the bit rate by reference to the preamblesignature, it may be determined by the MS 110 and indicated bysignalling in the message part 214.

As well as its application in a FDD system as described above, thepresent invention could be applied in other types of communicationsystem. For example, it could be used in a Time Division Multiple Access(TDMA) system provided that the uplink transmissions take place indifferent time slots to the downlink transmissions.

The embodiments described above relate to packet transmission. However,the same principles can equally well be applied to a system in whichcircuits are set up for data transmission.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the design, manufacture anduse of radio communication systems and component parts thereof, andwhich may be used instead of or in addition to features alreadydescribed herein. Although claims have been formulated in thisapplication to particular combinations of features, it should beunderstood that the scope of the disclosure of the present applicationalso includes any novel feature or any novel combination of featuresdisclosed herein either explicitly or implicitly or any generalisationthereof, whether or not it relates to the same invention as presentlyclaimed in any claim and whether or not it mitigates any or all of thesame technical problems as does the present invention. The applicantshereby give notice that new claims may be formulated to such featuresand/or combinations of features during the prosecution of the presentapplication or of any further application derived therefrom.

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

1. A radio communication system having a random access channel fortransmissions from a secondary station to a primary station, thesecondary station having means for requesting allocation of a randomaccess channel resource by transmission of an access preamble encodedwith a first signature selected from a first set of signatures and theprimary station having means for receiving the access preamble, fordetermining the first signature and for transmitting an accessacknowledgement encoded with a second signature selected from a secondset of signatures indicating whether the requested resource isavailable, wherein the choice of first signature provides furtherinformation regarding the resource allocation request and wherein eachsignature of the second set corresponds to a plurality of signatures inthe first set.
 2. A system as claimed in claim 1, characterised in thatnot all of the signatures comprising the first set of signatures aremutually orthogonal and in that any pair of signatures comprised in thefirst set of signatures have low cross correlation.
 3. A primary stationfor use in a radio communication system having a random access channelfor transmissions from a secondary station to the primary station,wherein means are provided for receiving an access preamble transmittedby the secondary station, which preamble is encoded with a firstsignature selected from a first set of signatures, for determining thefirst signature, for determining from the access preamble which randomaccess channel resource the secondary station requests to be allocatedand for transmitting an access acknowledgement encoded with a secondsignature selected from a second set of signatures indicating whetherthe requested resource is available, wherein the choice of firstsignature provides further information regarding the resource allocationrequest and wherein each signature of the second set corresponds to aplurality of signatures in the first set.
 4. A primary station asclaimed in claim 3, characterised in that means are provided fordetermining at which of a plurality of available time offsets the accesspreamble was transmitted, which offset provides further informationregarding the resource allocation request.
 5. A primary station asclaimed in claim 4, characterised in that means are provided forreceiving a contention resolution preamble, transmitted by the secondarystation in response to reception of the access acknowledgement, fordetermining at which of the plurality of available time offsets thecontention resolution preamble was transmitted, and for transmitting acontention resolution acknowledgement indicating whether the issecondary station has been granted access to the requested resource. 6.A secondary station for use in a radio communication system having arandom access channel for transmissions to a primary station, whereinmeans are provided for requesting allocation of a random access channelresource by transmission of an access preamble encoded with a firstsignature selected from a first set of signatures and for receiving anaccess acknowledgement encoded with a second signature selected from asecond set of signatures indicating whether the requested resource isavailable, wherein the choice of first signature provides furtherinformation regarding the resource allocation request and wherein eachsignature of the second set corresponds to a plurality of signatures inthe first set.
 7. A secondary station as claimed in claim 6,characterised in that the access preamble is transmitted at one of aplurality of available time offsets, which offset provides furtherinformation regarding the resource allocation request.
 8. A secondarystation as claimed in claim 7, characterised in that means are providedfor receiving an access acknowledgement from the primary stationindicating successful reception of the access preamble, and fortransmitting in response a contention resolution preamble at one of theplurality of available time offsets.
 9. A secondary station as claimedin claim 8, characterised in that use of any one of the plurality ofavailable time offsets results in the transmission of the preamble inadvance of the time at which it would be transmitted without the offset.10. A secondary station as claimed in claim 8 or 9, characterised inthat the time offsets used for transmission of the access preamble andcontention resolution preamble are the same.
 11. A method of operating aradio communication system having a random access channel fortransmissions from a secondary station to a primary station, the methodcomprising the secondary station requesting allocation of a randomaccess channel resource by transmission of an access preamble encodedwith a first signature selected from a first set of signatures and theprimary station receiving the access preamble, determining the firstsignature and transmitting an access acknowledgement encoded with asecond signature selected from a second set of signatures indicatingwhether the requested resource is available, wherein the choice of firstsignature provides further information regarding the resource allocationrequest and wherein each signature of the second set corresponds to aplurality of signatures in the first set.
 12. A method as claimed inclaim 11, characterised by not all of the signatures comprising thefirst set of signatures being mutually orthogonal and by any pair ofsignatures comprised in the first set of signatures having low crosscorrelation.
 13. A method as claimed in claim 11 or 12, characterised bythe access preamble being transmitted at one of a plurality of availabletime offsets and by the primary station determining its time offset,wherein the transmission time offset of the access preamble providesfurther information regarding the resource allocation request.